The present invention relates to a method and device for feedback controlling an air-fuel ratio of an internal combustion engine, which controls an air-fuel ratio of a air-fuel mixture formed inside a cylinder of an engine, to a target air-fuel ratio.
Heretofore, there has been known an air-fuel ratio feedback control device of a construction which corrects a fuel injection quantity based on a deviation between an air-fuel ratio detected by an air-fuel ratio sensor provided in an exhaust pipe and a target air-fuel ratio, so that an air-fuel ratio of an air-fuel mixture formed inside a cylinder of an engine coincides with a target air-fuel ratio (refer to Japanese Unexamined Patent Publication No. 6-108901).
However, a dead time due to exhaust gas propagation occurs up until the correction result of the fuel injection quantity based on the air-fuel ratio detected by the air-fuel ratio sensor becomes detected by the air-fuel ratio sensor.
Therefore, if a feedback control is performed without taking this dead time into consideration, then overshoot occurs, and it is required to set a feedback gain that can maintain a response characteristic while suppressing the overshoot.
Therefore, conventionally an optimum gain is obtained beforehand for each engine intake air quantity and engine rotational speed correlated with the dead time, and this optimum gain is stored in a map, and the gain corresponding to the intake air quantity and the rotational speed at that time is retrieved from the map, to be used in the feedback control.
Consequently, conventionally a large amount of man-hour is required in order to perform the gain adaptation. Moreover, a large amount of storage capacity is necessary for the map to store the gain for each operating condition. Furthermore, there is a problem in that in order to avoid overshoot, it is not possible to feedback control the air-fuel ratio at a high response characteristic.
The present invention takes into consideration the above problems, with an object of enabling correction of air-fuel ratio at a high response characteristic without needing to set a gain corresponding to a dead time, by providing a method and device for feedback controlling an air-fuel ratio of an internal combustion engine, which accurately estimates an air-fuel ratio of an air-fuel mixture formed inside a cylinder of an engine and feedback controls a fuel injection quantity from the estimation result.
In order to achieve the above object, the present invention is constructed such that an air-fuel ratio of an air-fuel mixture formed inside a cylinder is estimated, and an air-fuel ratio correction amount for correcting a fuel injection quantity is computed based on the estimated cylinder air-fuel ratio, while an air-fuel ratio to be detected by an air-fuel ratio sensor installed in an exhaust pipe is estimated based on the estimated cylinder air-fuel ratio, and the estimated cylinder air-fuel ratio is corrected based on the air-fuel ratio estimated to be detected by this air-fuel ratio sensor and an air-fuel ratio detected by the air-fuel ratio sensor.
With such a construction, the cylinder air-fuel ratio is estimated and the fuel injection quantity is feedback controlled so that the estimated cylinder air-fuel ratio coincides with a target air-fuel ratio, while the air-fuel ratio to be detected by the air-fuel ratio sensor is estimated based on the fact that the estimated cylinder air-fuel ratio is belatedly detected by the air-fuel ratio sensor, and from this estimation value and the air-fuel ratio actually detected by the air-fuel ratio sensor, an estimation error for the cylinder air-fuel ratio is corrected.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.